Elsevier

Catalysis Today

Volume 151, Issues 1–2, 15 April 2010, Pages 173-177
Catalysis Today

Use of conductive-diamond electrochemical oxidation for wastewater treatment

https://doi.org/10.1016/j.cattod.2010.01.058Get rights and content

Abstract

In this work, the more relevant figures about the use of conductive-diamond electrochemical oxidation (CDEO) in the treatment of industrial wastes are described. It is shown that it is a good technology to treat many industrial effluents with organic loads below 20000 mg dm−3, very efficient, robust and non-selective. It has been observed that the nature of the pollutant does not affect significantly the efficiency of the process. Conversely, organic load and current density show a marked influence in the results. The efficiency of the process increases with the organic content, and mass-transfer limitations seems to be the major drawback of the technology. CDEO is more efficient that electrolysis with other anodic materials, and its main advantage against other advance oxidation technologies is that it does not lead to the formation of oxidation-refractory intermediates. The combined action of direct electrolysis, hydroxyl radicals and that of other oxidants produced from the salts contained in the wastes seems to be the responsible of this good behavior. Two observations related to the production of hydroxyl radicals help to support this: the effect of the pH and that of the anodic potential on the efficiencies of the process.

Introduction

In the recent years, several works have been published concerning the application of conductive-diamond electrochemical oxidation (CDEO) for the treatment of aqueous wastes. Pollutants oxidized in the studies with synthetic wastes include cyanide [1], carboxylic acids [2], [3], alcohols, and aromatic and polyaromatic compounds [4], [5], [6], [7]. However, this list is even greater if the treatment of actual wastes is also considered. Thus, CDEO has been applied to the treatment of olive-oil mills [8], wooden manufacturing factories [9], [10], [11], surphactants [12] and even to the treatment of wastes consisting of merged effluents of different industries [13], [14]. In every case, a successful treatment has been obtained. Thus, it has been concluded that it is a robust and efficient technology which, in most cases, is able to achieve the complete mineralization of the organics contained in the wastes. The efficiency of the technology is very high, and it only seems to be limited by the transport of pollutants to the anodic surface.

Many approaches have been used to increase these efficiencies, such as the special design of cells [15], the combination of cathodically produced hydrogen peroxide with CDEO [16], or the use of ultrasounds [17]. All of them have shown good results, and have focused CDEO as a hot topic in industrial wastewater treatment and in the treatment of other wastewater such as those coming from soil remediation processes.

In addition, conductive-diamond electrodes show a great chemical and electrochemical stability, and an acceptable conductivity. The high overpotential for water electrolysis seems to be the more important property of conductive-diamond in its use in aqueous media. This electrochemical window is large enough to produce hydroxyl radicals with high efficiency, and this radical seems to be directly involved in the oxidation mechanisms that occur on diamond surfaces [18]. According to literature, direct oxidation models fit well the experimental data [19], [20], [21], [22], specially for non-chlorinated or nitrogenated substituted aromatics. However, it is known that in the electrochemical oxidation of wastewaters on conductive-diamond other oxidants are generated including persulphates [23], peroxophosphates [24], oxochlorinated anions [25] and hydrogen peroxide [26], depending on the waste composition and on the operation conditions. Thus, besides direct electro-oxidation on the surface and oxidation by means of hydroxyl radicals in a region close to the electrode surface, the oxidation mediated by other oxidants electrogenerated on the surface from the electrolyte salts should be taken into account, as it can complement the mechanisms of oxidation in this kind of electrochemical technology, and it contributes to increase the global oxidation efficiency.

Consequently, CDEO has shown better perspectives towards its application than other electrochemical oxidation technologies, and even that other advanced oxidation processes. In this context, the goal of this paper is to describe the more relevant figures of CDEO related to its use in industrial applications, and to point out the important role of the oxidation mechanisms that happen inside the electrochemical reactor and that shift CDEO as a very promising technology for the removal of organic pollutants in industrial wastes.

Section snippets

Wastewater characterization

In this work, both synthetic (propanol, phenol, hydroxybenzenes, chlorophenols, nitrophenols and dyes) and actual (pharmaceutical, olive-oil mills, chemical, petrochemical and door-manufacturing) wastewaters have been studied. In the case of electrochemical oxidation 5000 mg Na2SO4 dm−3 was used as supporting electrolyte.

Analytical procedure

The chemical oxygen demand (COD) was used to monitor the organic load of the wastes. It was determined using a HACH DR200 analyzer. Measurements of pH and conductivity were

Results and discussion

Fig. 1 shows the changes of the COD with the specific charge passed during the CDEO of four actual wastewaters. These effluents consist of aqueous wastes with a high concentration of organics, coming from the raw materials, intermediates and products of the different manufacturing plants (petrochemical, fine-chemical, door-manufacturing plants and olive-oil mills).

As it can be seen, and although the four wastes are very different in composition and organic load, the electrochemical process can

Conclusions

The main conclusions that can be drawn from this work are:

  • The electrochemical oxidation with conductive-diamond can be used to remove the organic content of a great variety of synthetic and actual wastewaters. Opposite to Fenton oxidation or ozonization, this technology does not lead to the formation of oxidation-refractory species and it allows diminishing the organic load of any effluent down to any required discharge limit.

  • The electrochemical oxidation of industrial wastes is strongly

Acknowledgment

The financial support of Spanish government through project CTM2007-60472/TECNO is gratefully acknowledged.

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